1. Field of the Invention
Aspects of the present invention relate to a secondary battery, and more particularly, to a secondary battery that prevents electrode tabs from breaking, and has improved weldability.
2. Description of the Related Art
A secondary battery can be repetitively charged and discharged, which differs from a dry cell that can only be discharged once. Secondary batteries have been actively developed, and are widely used, because they can be small and can have high-densities.
Low-density batteries, in which one battery cell is wrapped in a case, have been used in small-sized portable electronic devices, such as portable phones, PDPs, notebook computers, cameras, and the like. High-density batteries, in which a number of battery cells are connected, have been used to power motors, such as a motor in a hybrid automobile, etc.
Lithium secondary batteries have three times the operating voltage, as compared to conventional Nickel-Cadmium and Nickel-Hydrogen batteries, and also have higher energy density per unit weight. Accordingly, lithium secondary batteries are being used in an increasing number of applications.
Lithium secondary batteries can be divided into can-type and pouch-type, according to the shape of a case housing the electrode assembly. The can-type can be prismatic or cylindrical. Lithium secondary batteries can be classified as Lithium-ion, or lithium-polymer secondary batteries, according to the type of electrolyte included therein.
When a lithium secondary battery is over-charged, the electrolyte evaporates at an upper portion of the electrode assembly, thereby increasing resistance. Further, lithium can be precipitated, since a deformation occurs around the center of the electrode assembly. Heat can be locally generated, due to the increase of resistance at the upper portion of the electrode assembly, thereby increasing an internal temperature of the secondary battery. An internal pressure of the secondary battery can abruptly increase, due to the action of electrolyte additives, such as Cyclo-Hexane Benzen (CHB), benzophenone (BP), and the like, which easily dissolve, and generate gas during over-charging. The safety of a lithium secondary battery can be improved, by interrupting current flow at a cap assembly, to prevent further increases in the internal temperature or pressure of the battery.
A cylindrical secondary battery comprises a can, a cap assembly to seal the can, an insulating gasket to insulate the cap assembly from the can, and an electrode assembly wound in jelly-roll shape, contained in the can. An electrolyte is injected into the can. The electrode assembly comprises two electrode plates, and a separator interposed between the electrode plates, to prevent the electrode plates from shorting.
The electrode assembly is electrically connected to the can and cap assembly. Specifically, a negative electrode tab of the electrode assembly is connected to the can, and a positive electrode tab of the electrode assembly is connected to the cap assembly. When the positive electrode tab is connected to the cap assembly, the positive electrode tab should be extended for effective welding. When the cap assembly is combined with a top portion of the can, after being welded to the positive electrode tab, a portion of the positive electrode tab is bent, which is referred to as a bent portion. The bent portion can easily be cut, due movements of battery. The positive electrode tab may be broken, and may form a short circuit, thereby causing safety concerns.